Scanning probe microscopy refers generally to a class of high resolution techniques for studying surfaces at or near atomic resolution. Several different techniques which produce these results have been described in the prior art.
One of the first such techniques is scanning tunneling microscopy (STM), which utilizes a sharpened tip held in electrical contact (circa 0.1 to 1 nm) with a sample surface, and biased to produce a current between the tip and the surface. Current variations caused by differences in the distance between the tip and the surface may be plotted to yield a topographical representation of the surface. This technique is described in Binnig et al. (U.S. Pat. No. 4,343,993). In Binnig, the tip and the sample must be electrically conductive to allow current flow between them, and as such, limits the application of this technique. Another limitation is that the STM is sensitive only to the charge density at the surface of the sample.
A later variant of scanning probe microscopy was described by Binnig, Quate, and Gerber who reported the earliest atomic force microscope (AFM). (See: Binnig et al., Physics Review Letters, Vol. 56, page 930 (1986)). This early atomic force microscope, as further described in Binnig (U.S. Pat. No. 4,724,318), utilizes a small, diamond microprobe tip mounted on the side of a soft cantilever, so that the axes of the cantilever and the tip are substantially perpendicular to one another. The tip is brought into close proximity (0.1 to 1 nm) to the sample, with the cantilever disposed parallel to the sample, so that the repulsive forces between the tip and surface cause deflections of the cantilever.
In the Binnig AFM application, the tip is attached to a conductive cantilever which is interposed between the sample surface and a standard STM tip. A tunneling current is then maintained between the AFM cantilever and the STM tip, so that the sample need not be conductive. Changes in the current flow between these two elements provide a sensitive measure of the deflections of the AFM cantilever, and hence of the forces between the sample surface and the tip. More recent and more common AFM applications use other techniques, such as optical methods, to measure cantilever deflection.
In one mode of operation, changes in the deflection of the cantilever are measured as the tip is rastered over a sample. In practice, the tip scans the sample in very close proximity (&lt;1 nm) to the sample surface, so that the deflections of the cantilever are due to the repulsive forces between the atoms on the sample and the atoms on the apex of the tip.
As was immediately recognized by Binnig et al., a generally superior mode of operation utilizes dynamical techniques to reduce noise, and thereby increase sensitivity. By vibrating the cantilever perpendicular to the plane of the sample (i.e. vertically relative to a horizontally disposed sample) at the mechanical resonance frequency of the cantilever, noise is reduced by the quality factor of the mechanical resonator and by the narrow frequency range of the measurement. A possible disadvantage of this "resonance enhancement" configuration is that it is sensitive primarily to force gradients in the direction of vibration, the net force perpendicular to the sample surface (Z-direction) must still be determined from the static deflections of the cantilever.
Furthermore, a fundamental drawback of the Binnig AFM geometry is that the tip can only be vibrated in a direction that is substantially perpendicular to the sample surface. Thus, although forces between the tip and sample act in all directions of XYZ space, only the force gradients in the direction of oscillation (Z-direction) can be sensed by a cantilever that is oriented parallel to the sample surface; important information about the forces in the X- and Y-directions cannot be resolved.
Mate et al. (Phys. Rev. Lett., 59 pg. 1942 (1987)) describe a frictional force microscope for measuring lateral forces (see also Meyer and Amer, Appl. Phys. Lett., vol. 57 pg. 2089 (1990) and Marti et al., Nanotechnology, vol. 1 pg. 141 (1990)). The cantilever-tip configuration of these frictional force microscopes is similar to that of the Binnig AFM (with the cantilever axis oriented substantially parallel to the sample's surface), but lateral forces are measured from the torsional rotation of the cantilever about its axis. This configuration, however, has some fundamental drawbacks that limits its sensitivity to lateral forces. First, the cantilever spring constant is always stiffer against torsional rotation than it is against normal displacement. Second, it is generally difficult to excite torsional modes of vibration, hence no resonant enhancement of the lateral force resolution has been achieved with these microscopes. Finally, only forces in one lateral direction (for example, in the X-direction, but not the Y-direction) can be measured at a time.
Taubenblatt (Appl. Phys. Lett., vol. 54 page 801 (1989)) describes a different configuration for atomic force microscopy. The Taubenblatt microscope utilizes a vertically mounted STM tip, which can be made to vibrate laterally over the surface of a horizontally disposed sample. Changes in the vibrational frequency of the STM tip provide information about changes in the lateral atomic forces between the tip and the sample, thus rendering simultaneous STM and AFM images. The Taubenblatt STM/AFM microscope requires that the tip and sample be made from electrically conductive materials. Furthermore, the tip and sample must carry a possibly damaging electrical current. Finally, during imaging, the tip must be kept in close proximity to the sample surface, so it cannot be used for non-contact force microscopy.
Pohl (U.S. Pat. No. 4,851,671) describes a similar configuration for the measurement of lateral atomic forces. The Pohl AFM utilizes a rigid microprobe tip attached to (or etched from) a quartz crystal oscillator. In practice, the tip is brought into close proximity to the sample (&lt;1 nm), and the tip is vibrated laterally across the sample surface. Changes in the resonance frequency of the quartz crystal provide information about the force gradients at the surface in the direction of oscillation.
A fundamental drawback of the Pohl AFM is that the oscillator consists of a quartz crystal, which has a static spring constant of approximately 4.times.10.sup.6 N/m. Generally, such a "stiff" spring constant AFM requires complex instrumentation to achieve the sensitivity of a typical "soft" cantilever (0.01-100 N/m) AFM. Specifically, Pohl uses a sophisticated frequency counter, capable of resolving changes of 2 parts per billion in the frequency of the crystal, for sensitivity (2.times.10.sup.-12 N) comparable to that of a simple "soft" cantilever AFM. Furthermore, such frequency resolution requires relatively long integration times (about 1 second per point), which limits the scan speed to several minutes per line. Finally, because of the very high spring constant of the quartz crystal oscillator, the Pohl microscope does not interact detectably with the sample until the tip is in close proximity (within 0.1 to 1 nm) of the sample surface, thus the Pohl microscope is limited to the measurement of relatively strong interatomic forces.
Sometimes it is desirable to measure other types of forces from a sample. Abraham (U.S. Pat. No. 4,992,659) teaches an electro-magnetic force microscopy means and method whereby Lorentz forces arising from the interaction of a current interposed between the metallic conductive tip and a conductive sample surface are measured. Lorentz forces are those forces that arise when the current flow between the tip and sample of a STM system are deflected by a magnetic field.
Abraham's preferred tip is non-magnetic to eliminate extraneous forces created by a magnetic tip. When an STM current is interposed between the conductive tip and the conductive sample, Lorentz force (electro-magnetic) induced deflections of the vibrating tip can be measured. The strength of the force, and hence the magnitude of the deflection, is plotted using laser positioning instrumentation. Fundamental limitations of the Abraham Lorentz force microscope include that the sample must be conductive, and that a potentially damaging and invasive current must pass through the surface.
Furthermore, as in the other prior art techniques for contact AFMs described above, the Abraham microprobe tip must be placed at a distance of approximately 1 nm (or less) from the surface to be studied. To obtain a complete image, this close proximity must be maintained throughout the scan. Thus, to follow the contours of a sample, rapid motion of the tip assembly must be made to avoid catastrophic impingements (crashes) of the tip onto the sample. This indicates that only microscopically smooth regions of a surface may be investigated successfully, which is an obvious deficiency when the investigator desires to study an irregular surface.
Some improvements in the prior art have been made by operating standard (perpendicular cantilever-tip configuration) AFMs in non-contact mode. In non-contact mode, the AFM tip is held farther from the sample (1-100 nm), where a diversity of longer-ranged interactions such as magnetic, Van der Waals, or electrostatic dipole forces may be resolved. In general, these long-ranged forces are weaker and more dispersed than the inter-atomic forces measured by contact AFM, which thus has the advantage of diminishing the possibly destructive forces that the tip imparts on the sample, but demands that maximum sensitivity be available for near atomic resolution.
Any force microscope that incorporates a tunneling current between the sample and tip (such as the Taubenblatt AFM/STM or the Abraham near-field Lorentz magnetic force microscope) cannot be operated in non-contact mode. Moreover, any force microscope with limited force resolution (such as the Pohl oscillating quartz AFM) cannot be operated in non-contact mode. All prior art AFMs that have been operated in non-contact mode have embodied the original Binnig configuration of a tip that is substantially perpendicular to the cantilever, but this configuration has some fundamental drawbacks. First, the tip can only be vibrated in a direction substantially perpendicular to the surface of the sample, hence the primary contrast mechanism is due only to force gradients in the Z-direction. Second, information about the X- and Y-forces along the surface of the sample can only be obtained by using less sensitive torsional force sensing. Third, large variations in the force between the surface and the tip often cause uncontrolled deflections of the cantilever toward the sample surface, resulting in catastrophic tip-to-sample collisions (tip crashes). To reduce the incidence of tip crashes, cantilevers that are relatively rigid must be used, which limits the ultimate sensitivity of all force microscopes that are based on the original Binnig AFM configuration.
Although the prior art has addressed certain limitations in the art of scanning probe microscopy, none overcome all of the disadvantages. Accordingly, the present invention is directed toward providing to the art of force microscopy a novel means of producing high resolution analysis of a surface, either conductive or non-conductive, at a tip distance which will not contact (through electric current or inadvertence) the surface under study. A further goal is to provide a tip configuration which allows for measurement of both axial and multi-directional lateral forces, is effective in ambient conditions, is sensitive to small force gradients, and can be obtained using inexpensive laboratory instrumentation.